The central focus of the research herein is the development of new, highly site-selective reactions that can expand and complement the current state of the art in protein modification. A multidisciplinary approach has been developed that combines protein expression, purification, and characterization with the physical organic chemistry and organic synthesis techniques required for new reaction development. In particular, the reactions under study are designed to be orthogonal to existing lysine- and cysteine-based strategies, and thus can be used in concert with these tried-and-true methods. By targeting underutilized functional groups and by enhancing reaction selectivity between similar functionality, it is also anticipated that many entirely new avenues for protein labeling, protein immobilization, and de novo protein synthesis will emerge from these studies. Specifically, two powerful new strategies will be developed for the facile modification of tyrosine residues on protein surfaces. The first uses diazonium-coupling reactions to activate tyrosine residues, followed by a hetero-Diels-Alder reaction for further conjugation. Start to finish, this procedure takes as little as three hours to carry out, and can be used to couple alkenes and alkynes to tyrosines with absolute selectivity. The second strategy is a new three-component Mannich-based coupling reaction that can modify tyrosine residues with exquisite selectivity and simultaneously install two new functional groups. This reaction will be explored for as a new strategy for protein ligation. Transition-metal catalyzed reactions will also be explored, as they offer many opportunities for highly selective reactions and the activation of currently unmodifiable functional groups. In particular, the development of a metal carbene-based strategy for disulfide bonds will be explored as a means to modify antibody fragments and N-terminal methionine residues.